As resource companies evaluate increasingly more difficult polymetallic orebodies to assess their potential for exploitation, so does the metallurgical challenge increase. This is particularly the case for ores containing significant amounts of platinum group metals (PGMs), gold and silver together with valuable base metals such as nickel, cobalt and copper. Sulphide concentrates can present challenges to conventional processing by smelting, for example when barren sulphides such as pyrite and pyrrohotite contaminate the concentrate with significant amounts of iron and sulphur that have to be dealt with in an efficient and environmentally responsible manner.
In many cases there are potential technical, economic and environmental benefits from application of a robust hydrometallurgical processing route to these projects. These may include lower power costs, higher overall metal recoveries and lower SO2 and CO2 emissions to the atmosphere.
The novel process route described herein exemplifies significant developments that augment the process route described in international patent publication no. WO 99/60178 (the content of which is incorporated herein by reference), and which is known as the “Kell Process” (FIG. 1). This invention describes a novel way of treating the iron and sulphur, thereby removing concentrate quality constraints so that the flotation recovery of the valuable metal can be maximized. The Kell process route comprises the steps of:                leaching flotation concentrate (10) in a pressure oxidation sulphide leach (12) to dissolve base metal sulphides in the flotation concentrate and forming a sulphide leach filtrate (14) containing base metals (16) and a residue containing PGMs; and        separating the sulphide leach filtrate (14) from the residue; and        roasting (18) the residue to form a calcine; and        leaching (20) the calcine in a chloride leach to dissolve the PGMs into solution forming a chloride leach filtrate for PGM recovery (22) and a solid waste residue (24).        
The Kell process route has been successfully tested on several different sulphide concentrates, including those from the UG-2 chromitite horizon, Merensky reef, Platreef and North American polymetallic concentrates, achieving high 90s percent recoveries of separate Ni/Co/Cu and PGM solution streams.
The quantity and type of sulphide minerals in the flotation concentrate are present to varying extents, depending on factors such as ore mineralogy, flotation mass pull, relative flotabilities of the minerals, requirement to maximise overall metal recovery and other considerations. Barren sulphide minerals that do not bear valuable precious and base metals may be recovered into the flotation concentrate to a greater or lesser extent, depending on these factors. Hence, sulphur and iron levels in the feed concentrate may vary considerably. These barren sulphides, such as pyrite and pyrrohotite are difficult to depress in the flotation process without detrimentally reducing the recovery of the valuable metals to the concentrate. The iron and sulphur that the barren sulphides bring with them have to be disposed of in a cost-effective and environmentally acceptable manner.
It is an object of this invention to address this difficulty.